Amend javascript function for new site (Quick job)

Hi Guys,
I just need a tiny bit of javascript re-written,
all it does is check the a size has been selected in a form..
The trouble is on the old site it was a drop down, and on my newly designed site it is a radio buttons so I think that is interfering.. I need a JavaScript expert. :)

The original function:-

Code:

---

function checkSizes()
{
  for(i=0; i<document.main.elements.length; i++)
  {
    if ( document.main.elements[i].name.match(/^colour_(.*)$/) && document.main.elements[i].value ) {
      if ( document.getElementById("size_" + RegExp.$1).value == '' ) {
        alert ('Please choose a size for all your items');
        return false;
      }
    }
  }
}

---

The page in which I want the function to occur:-
Example Product Page

The form specifics:-

Code:

---

<form action="basket.ghtml" method="post" name="main" target="_self" onsubmit="return checkSizes()">

[_foreach [_own Size_]_]
<div class="buttons">
<input type="radio" name="size_[_own ID_]" value="[_value_]" class="styled" />                                                                                <div class="label">                                                                                                        <span class="nohover">[_value_]</span>
<span class="onhover"></span>
</div>
</div>                                       
[_/foreach_]

<input type="submit" name="action" value="Add to Basket" class="button" />

</form>

---

So can anyone either edit my code or create a new piece of JavaScript to perform this function?
Thanks for your time.
Best Regards,
Pete